bump mkdocs, no longer need to pin jinja2

Addresses https://github.com/encode/django-rest-framework/security/dependabot/11

Author: peterthomassen

requirements/requirements-documentation.txt

@@ -1,6 +1,5 @@
 # MkDocs to build our documentation.
-mkdocs==1.2.4
-jinja2>=2.10,<3.1.0 # contextfilter has been renamed
+mkdocs==1.6.0
 
 # pylinkvalidator to check for broken links in documentation.
 pylinkvalidator==0.3